In a nonvolatile semiconductor memory, an electron is sent through an oxide insulation layer to a floating gate in a process for storing (programming) data to an internal cell. Furthermore, an electron is removed from the FG (Floating Gate) through the oxide insulation layer in a process for erasing (erase) the stored data.
A nonvolatile semiconductor memory comprising this kind of data storage mechanism puts a load on the oxide insulation layer at each programming and erase process. Repeating this load creates multiple electron traps and positive hole traps inside the oxide insulation layer.
The nonvolatile semiconductor memory measures a threshold voltage, which fluctuates in accordance with the charge (number of electrons) inside the FG, to determine the recorded data. For this reason, when electron traps and positive hole traps are formed inside the oxide insulation layer as mentioned hereinabove, the electrons in these electron traps and positive hole traps give rise to residual voltage, and this voltage causes the FG threshold voltage to fluctuate, making it impossible to accurately determine the recorded data.
The threshold voltage fluctuations resulting from electron traps and positive hole traps like these are known to be a function of the control interval and the cumulative number of times that programming and erasing are executed. Furthermore, changes in the data storage time characteristics (hereinafter, referred to as retention characteristics) in accordance with the fluctuation of the threshold voltage is also known.
Since a charge that has accumulated in the FG will change in accordance with the passage of time, the data storage period of a nonvolatile semiconductor memory is limited. In addition, the data storage period is further shortened by the fluctuation of the threshold voltage resulting from the above-mentioned electron traps and positive hole traps.
A semiconductor storage apparatus that uses a nonvolatile semiconductor memory as the storage medium (hereinafter, referred to as the storage apparatus) must satisfy the data storage period criteria stipulated in the apparatus specifications. The storage apparatus predicts the deterioration of the oxide insulation layer based on the number of times that data is programmed and erased, and in a case where it is predicted that the data storage period of a specific area of onboard nonvolatile semiconductor memory will not meet the criteria, makes this storage area unusable.
To ensure that operation can continue even when an unusable area such as this has occurred, the storage apparatus is generally equipped with spare storage areas in excess of the total amount of stored data of the apparatus, and exercises control such that this spare storage area is allocated in place of the above-mentioned area that has been determined to be unusable so as to constantly maintain a fixed storage area. However, when this allocated spare area is exhausted, the storage apparatus loses its ability to record new data and shuts down.
Therefore, to realize a long-life storage apparatus, it is necessary to hold the fluctuation of the threshold voltage in accordance with the electron traps and positive hole traps in check, and to prevent the data storage period of a storage area from being shortened.
For example, Patent Literature 1 discloses control for lessening the effects of these electron traps and positive hole traps. In the Patent Literature 1, there is disclosed a storage apparatus, which makes an area whose retention characteristics have worsened unusable for a fixed period, reduces the electron traps and position hole traps, and reuses the storage area (Patent Literature 1).